Smooth muscles, which surround the periphery of hollow organs, contract to change organ shape or maintain tension to fix the shape and thereby control the flow of vital fluids, which are essential to the normal functioning of the cardiovascular, respiratory, digestive, and reproductive systems. If the regulation of smooth muscle contraction does not function properly, it could contribute to such diseases as high blood pressure, asthma, and premature birth. The goal of our work is to understand the molecular basis of the normal regulation of contraction. Smooth muscle contraction is primarily regulated by the Ca2+ controlled phosphorylation of myosin in the thick filament. However there is not a strict coupling between phosphorylation levels and the level of the resulting contractile force. Evidence indicates that there is additional regulation in the actin thin filament possibly involving tropomyosin (Tm). However the mechanism of this function is poorly understood. The long-range goal of this project is to uncover the molecular mechanisms whereby Tm, in concert with other thin filament proteins, regulates smooth muscle contraction. The main hypothesis of this proposal is that thin filament regulation occurs mainly by controlling the movement of Tm on the thin filament by myosin in the thick filament and by the other thin filament proteins, caldesmon and calponin, which are in turn regulated by phosphorylation and Ca2+binding proteins. This will be tested by monitoring Tm's position, and movement by measuring the Tm-actin distances as a function of myosin, caldesmon and calponin by fluorescence resonance energy transfer and correlated with actomyosin ATPase activity, an in vitro analogue of contraction. The results of these studies, which will be conducted on reconstituted thick and thin filaments, will help to further our understanding of the switching on/off of smooth muscle contraction and of smooth muscle's unique ability, especially vascular muscle, to maintain tension, and thus organ shape, at the cost of very little energy. These studies will compare myosin from vascular and gastrointestinal smooth muscles in order to better understand the ability of vascular muscle to maintain this tension.